<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .bigBox {
            height: 100vh;
            display: flex;
            flex-direction: column;
            background-color: red;
        }
        
        .header {
            height: 150px;
            background-color: aqua;
        }
        
        .content {
            background-color: blue;
            /* flex-grow: 1; */
            flex: 1;
        }
    </style>
</head>

<body style="margin: 0px;">
    <div class="bigBox">
        <div class="header">header</div>
        <div class="content">
        </div>
        <div class="content">
        </div>
    </div>
</body>

</html>